Basic linear voltage regulator guide
Most projects and electronics designs require stable voltage power supplies to operate properly. This guide will cover how to use linear voltage regulators to get a stable voltage supply from a higher "unstable supply".
Most prototypes we work on use either 5V or 3.3V power supplies that are either powered directly from the USB 5v line or from a higher voltage DC source.
This guide is a very basic tutorial on how to use voltage regulators. The subject of power supplies and voltage regulators is actually a lot more complex, and requires years of study to master. With that in mind, this guide covers the most common linear voltage regulators used in our projects.
Principle of operation
Linear voltage regulators work on the principle of negative-feedback loop amplifiers. A stable reference voltage is compared to the output of the voltage regulator, and that output is decreased or increased, to match the reference voltage.
We will also cover two important specifications of linear voltage regulators to which you need to pay attention for your power supply to work.
There is loads more information on this subject than we explain here, so if you're interested check out the links below.
Drop out voltage
Drop out voltage is basically the margin by which the input voltage must be higher then the output voltage at all times. So if your input voltage has a minimum of some 4.9V, this minus the dropout voltage is the maximum stable output voltage the regulator can provide.
Vout(max) = Vin(min) - Vdo
Or to put it more into perspective. The input voltage must be higher then the output voltage by a margin of at least the drop out voltage.
Vin(min) = Vout + Vdo
Maximum power dissipation
Another important factor with linear voltage regulators is the power dissipation. This is the amount of heat that is dissipated by your power supply. It is calculated with the following formula:
Pd(max) = (Vin(max) - Vout) * Iout(max).
Since power dissipation (heat) in linear regulators is a must, some regulators will require heatsinks to function within their parameters.
To get the highes safe power dissipation your regulator can stand, you need to consult its datasheet. The specification you are looking for is the Thermal resistance for Junction-to-Ambient, for the selected package.
This number tells you by how many degrees Celsius the regulator will heat up with each Watt of power dissipation. This coupled with the maximum operating temperature, and the maximum expected ambient temperature can be used to figure out how many watts you can dissipate safely.
Use this formula: Pdm = (Tm-Ta)/Rj. Where Tm is the maximum operating temperature for the device, Ta is the maximum expected ambient temperature, while Rj is the Junction to Ambient coefficient.
- Rj = 50 (TO-220 case)
- Tm = 125C (TO-220 case)
- Tm = 40C maximum expected ambient temperature
Pdm = (125-40)/50 = 1.7W.
In this example it is safe to dissipate 1.7W, but it's always a good idea to keep below this number by at least a third. We don't recommend you dissipate more then 1W in this setup.
- Maximum expected ambient temperature, is the maxim temperature at which the air around the device is. Keep in mind that if the device is enclosed, and even if it isn't, the air around it will heat up.
When using heatsinks, everything is the same, with the exception that you use the junction-to-Case Thermal resistance specification from the datasheet. This added to the Thermal resistance of the heatsink, gives you the total Thermal resistance you can use in the formula above instead of Rj.
Voltage regulator examples
Here we will give examples of power supply circuits using some of the most common linear voltage regulators. As with any electronics design, datasheets are your friends, and most of the time you can find in them example circuits that fit your application.
Fixed voltage linear regulators
Fixed voltage linear regulators are simple to use, usually only requiring one or two external capacitors to give you a stable power supply. In our experience, the most commonly used fixed voltage regulators are the 78XX series. Where the XX stands for the stable output voltage you need (eg a 7805 provides a 5V output voltage).
They have a drop-out voltage of some xx volts, so make sure the input voltage is at least that much higher than the required output voltage.